Technical Field
The present invention relates to a method for manufacturing a rare-earth magnet.
Background Art
Rare-earth magnets containing rare-earth elements such as lanthanoide are called permanent magnets as well, and are used for motors making up a hard disk and a MRI as well as for driving motors for hybrid vehicles, electric vehicles and the like.
Indexes for magnet performance of such rare-earth magnets include remanence (residual flux density) and a coercive force. Meanwhile, as the amount of heat generated at a motor increases because of the trend to more compact motors and higher current density, rare-earth magnets included in the motors also are required to have improved heat resistance, and one of important research challenges in the relating technical field is how to keep magnetic characteristics of a magnet operating at high temperatures.
Rare-earth magnets include typical sintered magnets including crystalline grains (main phase) of about 3 to 5 μm in scale making up the structure and nano-crystalline magnets including finer crystalline grains of about 50 nm to 300 nm in nano-scale. Among them, nano-crystalline magnets capable of decreasing the amount of expensive heavy rare-earth elements to be added or without such heavy rare-earth elements added while making the crystalline grains finer attract attention currently.
The following briefly describes one example of the method for manufacturing a rare-earth magnet. In a typical method, for instance, Nd—Fe—B molten metal is solidified rapidly to be fine powder (magnetic powder), while pressing-forming the fine powder to be a sintered body. Hot deformation processing is then performed to this sintered body to give magnetic anisotropy thereto to prepare a rare-earth magnet (orientational magnet). The hot deformation processing is performed by extrusion such as backward extrusion or forward extrusion, or upsetting (forging), for example. Patent Document 1 also discloses a method to orient crystalline grains through hot deformation processing to manufacture a rare-earth magnet having high degree of magnetization and high coercive force.
Herein the hot deformation processing is performed by placing a sintered body in a cavity of a forming die made up of a die and a lower punch and/or an upper punch sliding in the die, for example, and hot-pressing the sintered body while sliding the upper punch, for example. At this time, glass-based lubricant or lubricant containing the mixture of glass-based lubricant (e.g., glass powder) and graphite powder is used as lubricant that can be used in a high-temperature atmosphere as well, and such lubricant is applied or sprayed on side faces of the die or the punch defining the cavity for hot deformation processing.
Such hot deformation processing, however, has the problem that the glass-based lubricant changes to liquid phase during the processing, so that the viscosity of the lubricant applied or the like on side faces of the die and the punch facing the cavity decreases and the lubricant flows down, thus causing a breakage of the film and failing to exert sufficient lubricity. This makes frictional force different between an area where the lubricant flows down and a region where the lubricant remains on the side faces facing the cavity, and makes pressing force acting on the sintered body different therebetween. In this way, non-uniform pressing force acts on the sintered body, so that deformability also varies from one place to another (uniform processing strain cannot be given), and a rare-earth magnet manufactured has different magnetic performance from one place to another.
For instance, in hot deformation processing to give deformation at the draft of 70% to a sintered body in the temperature atmosphere at 650° C., high-viscosity lubricant of about 1×103 Pas is required so as not to flow down from the cavity face. Although glass-based lubricant to meet this condition can be prepared, then it is difficult to apply or the like such high-viscosity glass-based lubricant to the cavity face, and so this cannot be said a practical method.
Another possible method is to readjust the processing conditions of hot deformation processing, including strain rate, pressing load and processing temperature to find the conditions to suppress flowing-down of glass-based lubricant. Such factors of strain rate, pressing load and processing temperature, however, are all important for the degree of orientation of a magnet, and so it is not easy to readjust these factors.